Preserving the environment goes above all through the recognition of polluting agents, a list stretching out every year. Carbon dioxide is a by-product of many industrial chemical reactions and is considered polluting only of late. Until recently still, it was rather seen as a consequence of no gravity from the combustion of carbonated compounds such as coal and petroleum, or else, as a desired final product in the control of gaseous emissions.
Accumulation of evidences as to the participation of CO2 in the greenhouse effect responsible for earth's global warming revealed a problematic of extreme urgency. On this account, international engagements proposed in Kyoto (Kyoto protocol, 1997) have commanded for active research in reducing greenhouse effect gases, particularly CO2.
Amongst the finger pointed industries, cement fabrics account for 8% of the CO2 planetary anthropic emissions. Portland cement is made of lime, alumina, iron and silica. This mixture is pulverized and fused together by burning at high heat (more than 1,400° C.) in a rotary kiln. Combining the resulting material, called “clinker”, with gypsum, results in the production of a fine powder known as Portland cement. In the production process, carbon dioxide emissions come from fossil fuel combustion and a calcination stage of limestone (CaCO3).
Portland cement is the binding agent that enables the formation of concrete, which will also include aggregates, air and water.
Portland cement is the result of a chemical combination of calcium (usually from limestone), silica (clay, sand and shale), alumina (bauxite), iron (ore) and small amounts of various chemicals called admixtures to which gypsum is added at the last grinding in order to control the cement setting process.
The production of one tonne of cement (2000) requires close to 3 500 tonnes of raw materials. Lime and silica account for approximately 85% of the weight.
Two main categories of production processes are in use in this industry: the dry process kiln (70% in the U.S.) and the wet process kiln (older). For both processes, the first step is similar. The raw materials are crushed for reduction to about 3 inches or smaller. In the wet process, the raw materials, properly proportioned, are then ground with water, thoroughly mixed in the form of slurry. In the dry process, raw materials are ground and mixed in a dry state. In both processes, the slurry or the dry material is fed to a rotary kiln inclined slightly form the horizontal where it is heated to more than 1,400° C.
A rotary kiln is frequently as much as 12 feet in diameter and 400 feet long.
The “raw meal” is fed into the higher end of the kiln and progresses to the lower end, moved by the rotation. On its way down, the “raw meal” is facing a burner blowing its flame under an upward forced draft.
At the lower end, the new substance, called clinker, is formed in pieces about the size of marbles. The heat is recuperated from the cooling of the red-hot clinker and returned to the kiln. The clinker remains to be ground into fine powder to which some gypsum is added to form cement.
Cement is then put in bags or carried in bulk to ready mix plants. Concrete is produced by mixing cement with fine (sand) or coarse aggregates (gravel or crushed stone) and water. Often, some chemicals are added to control setting time and plasticity.
The first important reaction to occur is the calcining of limestone (calcium carbonate CaCO3) into lime (calcium oxide CaO) and carbon dioxide (CO2) at a temperature of 1 650° F. (900° C.).
The second reaction is the bonding of calcium oxide and silicates to form calcium silicates. Small amounts of calcium aluminate and calcium aluminoferrite are also formed. The relative proportions of these compounds determine the key properties of the resultant Portland cement and the type classification.
Cement Kiln Dust (CKD)
Cement Kiln Dust (CKD), a fine granular material generated during cement production, is carried up by the combustion gases. The CKD is normally removed and collected by an air pollution control system. The collected CKD which is mainly made up of lime (CaO), K2O, SO3, silica (SiO2) and alumina (Al2O3) may be returned into the process or stocked.
The US Environmental Protection Agency published a report on the forseeable costs of conforming to future regulations on CDK dust (EPA, 1998). Among the interesting information published in this report, is the ratio of CDK production relative to clinker production depending on Kiln type (Table 1). As it can be appreciated, wet processes generate a greater production of CKD (between 11 and 17%).
TABLE 1Average CKD production vs clinkerproduction depending on Kiln typeMean CKD/clinkerKiln TypeproductionNon-hazardous fuel kilnDry process0.060Dry preheating0.024Wet process0.107Hazardous fuel kilnDry process0.061Dry preheating0.038Wet process0.166
The net CKD refuse (excluding CKD regenerated during the process) output by 110 US cement plants is approximately 3,3 million tonnes per year.
In the above-mentioned EPA study, the baselines and the costs relating to conforming to future regulations were calculated for every affected plant. The total before-tax baseline costs, the initial conformity costs and the annual conformity costs respectively amount to 54,9; 98,5 and 43,7 millions US$. Out of 110 US plants, 68 (62%) would incur average annual costs of 646 000 US$, translating to 19,96 US$ per CKD tonne and 1,12 US$ per cement tonne produced. The total annual income for these 68 cement plants is estimated at 2,945 billion US$. The cost of conforming to regulations would therefore represent 1,5% of their income.
CO2 emissions from cement Portland production
CO2 emissions produced by cement production come from two sources:                1. Fossil fuel combustion required to heat the raw meal—6 millions BTUs/tonne of cement produce ¾ tonne of CO2.        2. Calcining of limestone into lime—½ tonne of CO2/tonne of cement.        
In Canada, one tonne of cement generates 0.5 tonne of CO2 from limestone conversion and about 0.35 tonne of CO2 from the combustion of fossil products, for a total of 0.85 tonne of CO2 for each tonne of cement produced.
The very high temperatures required for cement production allow for burning all sorts of materials such as solvents, paint, and tires. At these temperatures, the combustion is complete and pollution is reduced to a minimum.
The methods already known in the art for reducing CO2 emissions are the improvement of the energy efficiency of the process, the replacement of high carbon fuels by low carbon fuels, the use of waste as alternative fuel, the use of industrial waste containing lime and the use of fly ash to replace cement (15 to 35%) in concrete. Fly ash is a by-product of the combustion of pulverized coal in thermal power plants.
Literature abounds with a good number of processes destined to reducing carbon dioxide emissions by the cement industry. U.S. Pat. No. 6,240,859; GB587101; U.S. Pat. No. 5,744,078 propose above all an improvement of combustion efficiencies and recycling of fly ashes as cement additive. Patent DE4032557 suggests a method of handling aggregates and cement claiming improved energetic efficiency and in this way, preventing, CO2 discharge by cement fabrics. Patent JP09-110485 uses absorption and volatilization of CO2 to produce energy. Finally, European patent WO9966260 advises reutilization of industrial exhaust gas, thus achieving improved combustion efficiency.
Japanese patent 10-130045 discloses cement waste recycling in the fabrication of regenerated cement, by the way claiming a reduction of CO2 emissions in the environment. As a matter of fact, combined recycling of CO2 and manufacture residues is often invoked as a means for reducing cementous polluting waste (JP2000-239670; JP2000-119049; JP09-168775). In some configurations (U.S. Pat. No. 6,187,277; JP07-068164; JP2000-300983), recycled solid materials can be directly used for carbon dioxide fixation of industrial gas exhausts. Another Japanese patent (JP10-314758) was-conceived so that CO2 from the gaseous effluent of ceramic industries can be eliminated by introduction of residual water in cement plants.
CA1143910 describes a method allowing the use of CO2 from a waste flue gas and some waste material for manufacturing an asbestos free fibre reinforced cement. A European untitled document (GB103290) also expects recycling of CO2 for Portland cement manufacturing, in the same way as patent GB384060, which particularly uses a simple valve to isolate produced CO2 from other flue gases during the process.
Another way of reducing industrial CO2 emissions consists of transforming this gas to synthesis products of commercial value. Patent JP2000-072981 suggests carbon black production by utilizing exhaust gas in cement production through CO2 catalytic fixation whilst another (JP58-208117) allows for the manufacture of liquefied carbon dioxide from the waste gas of cement calcining kiln. Patent JP06-263665 deals with catalytic hydrogenation of carbon dioxide gas exhausted from cement factories for the production of chemicals such as methane and methanol.
A totally different approach preaches cement admixtures and compositions giving low loads on the environment and discharging smaller amounts of carbon dioxide, as it is the case for patents FR2669918 and JP2001-039751.
Currently, many scripts emanate from scientists with the purpose of reducing CO2 emissions in general. Sinking CO2 in the seawater over the deep ocean floor (U.S. Pat. No. 5,364,611; U.S. Pat. No. 5,304,356; U.S. Pat. No. 5,293,751) persists amongst the main considerations despite the many uncertainties and inconveniences associated with this method. Chemical fixation (CA 2352969) and selective adsorption of CO2 (U.S. Pat. No. 5,667,561; JP02-000699) also stand amongst favoured methods in the literature. Membrane separation (JP11-235999) and chemical conversion in ether or methanol (U.S. Pat. No. 6,248,795) offer interesting possibilities as well. One cannot help, but conclude exhaust gas absorption (U.S. Pat. No. 6,117,404; CA2255287; U.S. Pat. No. 5,888,256) remains one of the most promising methods and the very solubilization of concerned gases constitute the corner stone of all innovations in that field.
With this perspective, carbonic anhydrase has already been used for waste water treatment (U.S. Pat. No. 6,110,370) and recently, for reducing CO2 emissions in enzymatic photobiorectors (U.S. Pat. No. 5,614,378). Carbonic anhydrase is a highly reactive enzyme observed in most animal and vegetal species and thus, readily available. Trachtenberg (U.S. Pat. No. 6,143,556; CA 2222030) describes a system for gas processing with carbonic anhydrase without suggesting any specific application for cement fabrics. Michigan University introduced a photobioreactor for the conversion of CO2 with carbonic anhydrase, however destined to medical use as an artificial lung for life support (WO 9200380; U.S. Pat. No. 5,614,378).
U.S. Pat. No. 6,258,335 unveiled a catalytic process for carbon dioxide removal of ice exhaust by chemical fixation. This technology includes the optional use of carbonic anhydrase without particular consideration for cement plants. U.S. Pat. No. (4,743,545) exposed a bioreactor with hollow beads including catalyst, the latter being carbonic anhydrase if needed.
U.S. Pat. No. 4,602,987 and U.S. Pat. No. 4,761,209 disclose a method for extraction and utilization of oxygen from fluids. This system conceived for oxygen recovering especially includes a step for carbon dioxide removal by carbonic anhydrase ( ).
EP0991462; AU7753398; WO9855210; CA2291785 in the name of the Applicant propose a countercurrent packed column bioreactor for treating carbon dioxide. In this process, carbonic anhydrase is used immobilized or in a free state.
Although a great deal of efforts have been made to reduce in general, the is greenhouse effect caused by the emissions of CO2, there is still a need for further improvements and that mainly in the field of cement clinker production.